1. Field of the Invention
The present invention is directed to a process for chain terminating high molecular weight polyurethanes by incorporating terminal aromatic amino groups. The present invention is also directed to the products of this process and compositions containing these products and polyisocyanate crosslinking agents.
2. Description of the Prior Art
The production of high molecular weight polyurethanes in solution has long been known; the products have acquired a significant market in the field of adhesives, coatings and lacquers by virtue of their outstanding properties.
Different alternatives for producing polyurethane solutions of the type in question have been described in numerous patents, for example in DE-A No. 25 00 921. The various approaches have been adopted with a view to obtaining storable solutions in the most efficient possible manner. All of these publications are directed to the problem of avoiding isocyanate excesses in the solution ultimately obtained without uncontrollably altering the solution, while at the same time enabling the solution viscosity of the polyurethanes to be specifically and rapidly adjusted to the level required for application of the polyurethanes. Although storable PUR-solutions can be obtained by reacting a mixture of compounds containing NCO-reactive groups in stages with polyisocyanates until the required viscosity is obtained, while at the same time avoiding isocyanate excesses, the process involved is of little interest from a commercial point of view due to the long reaction time towards the end of the reaction, particularly where polyols are used.
Less time-consuming processes are known which are suitable for more efficient production. These processes involve small isocyanate excesses which, on reaching the required viscosity, are reacted with an excess of diols, alkanolamines (GB-PS No. 1 351 773); alkanolamines and then epoxides (DE-A No. 25 01 987); monoalcohols (CA-PS No. 888 781); monoamines (DE-A No. 19 06 555); or monooximes (DE-A No. 31 42 706), in order to obtain storable solutions. However, when amines are used, any excess amino groups have to be reacted off, for example with monoisocyanates according to DE-A No. 25 02 921, because otherwise unstable solutions would be formed. Polyurethane products such as these are applied from solution as coatings either by drying or by coagulation (so-called one-component polyurethanes).
However, if special properties (such as solvent resistance, surface hardness or temperature stability) are required, crosslinking agents (such as polyisocyanates) are added to the polyurethane solutions before their application to produce a crosslinked polyurethane having the required properties on the substrate. The disadvantage of this procedure is that certain conditions, which are often difficult to maintain in practice, have to be satisfied if optimum properties resulting from crosslinking are to be obtained. For example, one essential requirement is the avoidance of small quantities of moisture in the PU-solutions. In some cases this necessitates expensive drying of solvents and additives (such as pigments, etc. which often contain water) or the addition of drying agents which may have an adverse effect. The mixture of polyurethane solutions and polyisocyanate crosslinking agents also has to be sufficiently stable in storage or stable at least during the processing phase. Accordingly, polyurethane solutions of the type obtained by reacting NCO-prepolymers with excess quantities of aliphatic diamines, hydrazine or polyhydrazide compounds (for example in accordance with DE-PS No. 1 157 386) are not suitable for the proposed crosslinking application because they react almost instantaneously upon the addition of polyisocyanates. It has also been proposed to use chain-terminating agents, such as aminoalcohols or diol amines. Although aminoalcohols react quickly through their primary or secondary (aliphatic) amino group, the remaining hydroxyl groups show very little reactivity and necessitate elevated crosslinking temperatures and/or increased quantities of crosslinking agent. This often leads to inadequately crosslinked products because secondary reactions of the NCO-groups, for example with water, can have a detrimental effect.
Accordingly, the object of the present invention is to provide suitable polyurethane solutions which do not have any of the disadvantages referred to above and which, in particular, may be crosslinked with polyisocyanates, even if the solution has a high moisture content. The crosslinking reaction is intended to take place easily, even at room temperature or at relatively low crosslinking temperatures, especially from solvent-free, polyurethane/polyisocyanate crosslinker mixtures. Further advantages of the invention will become apparent from the following discussion and from the results of the examples.
The main improvement in the production of readily crosslinkable polyurethanes arises from the fact that, initially, the polyurethanes are synthesized from the usual starting materials. After a relatively high molecular weight polyurethane is formed, a chain-termination reaction with certain diamines is conducted to avoid any further significant increase in molecular weight. As a result terminal aromatic amino groups are introduced into the polyurethanes where they serve as sites for further crosslinking reactions which take place easily and safely. The polyurethanes according to the invention contain certain quantities of chemically incorporated, terminal aromatic amino groups.